1. Field of the Invention
The invention relates to a hydraulic collet assembly, more particularly to a hydraulic collet assembly with a valve unit for relief of pressure in a fluid injection passage in the hydraulic collet assembly.
2. Description of the Related Art
Referring to FIG. 1, a conventional hydraulic collet assembly is shown to include a machine body 31, a stationary wall body 32 disposed in the machine body 31 and formed with a fluid injection passage 321, a rotary shaft 33 disposed coaxially through the stationary wall body 32 and cooperating with the stationary wall body 32 to define a clearance 300 therebetween, a bearing unit 331 disposed between the stationary wall body 32 and the rotary shaft 33 to facilitate rotation of the rotary shaft 33 relative to the stationary wall body 32, a tubular slidable seat 35 disposed in and cooperating with the rotary shaft 33 to define a piston-receiving chamber therebetween, a piston 34 disposed movably within the piston-receiving chamber and cooperating with the rotary shaft 33 to define a gap 34G therebetween, and a collet 36 disposed in the tubular slidable seat 35 and coupled to the piston 34 for synchronous movement therewith.
The rotary shaft 33 is formed with a fluid channel 332 in fluid communication with the fluid injection passage 321 in the stationary wall body 32 and extending to and in spatial communication with the piston-receiving chamber. The collet 36 has clamping jaws confining an open mouth. When high-pressure hydraulic oil is injected into the gap 34G through the fluid injection passage 321 and the fluid channel 332, the collet 36 moves together with the piston 34 toward the fluid channel 332 in such a manner that the open mouth in the collet 36 is reduced in cross-section so as to clamp a rod-shaped workpiece (not shown) therein.
Some disadvantages that result from the use of the aforesaid conventional hydraulic collet assembly are as follows:
During the clamping operation of the workpiece in the collet 36, the high pressure oil is introduced into the bearing unit 331 through the clearance 300 formed between the rotary shaft 33 and the stationary wall body 32. Since the fluid injection passage 321 and the fluid channel 332 have to be kept under a high pressure state so as to maintain the collet 36 in the clamping state, the pressure of the hydraulic oil accumulated in the clearance 300 is relatively high. As such, friction force between the stationary wall body 32 and the rotary shaft 33 is relatively large during rotation of the rotary shaft 33, which may result in an undesirable increase in the temperature of the bearing unit 331 and the collet 35, and a reduction in the viscosity of the hydraulic oil in the bearing unit 331, thereby shortening the service life of the components of the conventional hydraulic collet assembly.
Therefore, the object of this invention is to provide a hydraulic collet assembly with a valve unit for relief of pressure in a fluid injection passage so as to prolong the service life of the components of the assembly.
Accordingly, a hydraulic collet assembly of the present invention includes: a machine body; an annular stationary wall body disposed in the machine body and formed with a fluid injection passage therein; a hollow rotary shaft co-axially extending through the stationary wall body, defining a piston-receiving chamber, and formed with a fluid channel extending to and in spatial communication with the piston-receiving chamber; and an annular piston movably received in the piston-receiving chamber in the rotary shaft in such a manner that the piston moves away from the fluid injection passage upon injection of a high pressure hydraulic oil into the piston-receiving chamber through the fluid injection passage and the fluid channel. The piston defines a collet-receiving chamber therein. A collet is movably received in the collet-receiving chamber, and is coupled to the piston for synchronous movement therewith. The collet has clamping jaws confining an open mouth that is reduced in cross-section when the piston moves away from the fluid channel. A bearing unit is disposed between the stationary wall body and the rotary shaft. A valve unit is disposed between the stationary wall body and the rotary shaft, and includes a tubular valve sleeve sleeved on the rotary shaft. The valve sleeve includes an annular wall body formed with a valve opening therein, a valve chamber extending from the valve opening, a valve inlet in fluid communication with the fluid injection passage and the valve opening, and a valve outlet in fluid communication with the valve chamber and the fluid channel. The valve unit further includes a check valve movably mounted in the valve chamber and operable to move between an open position, in which the valve opening is opened, thereby permitting fluid communication between the fluid injection passage and the fluid channel through the valve inlet and outlet, and a closed position, in which, the valve opening is closed by the check valve, thereby preventing fluid communication between the fluid injection passage and the fluid channel.